1. Field of the Invention
The present invention relates to a commutator motor used as a motor for an electric power steering assembly for assisting the steering force of an automotive steering wheel, for example.
2. Description of the Related Art
FIG. 10 is a perspective showing use of a motor for an electric power steering assembly which is a conventional commutator motor. This commutator motor 1 is connected to a gear box 3 mounted to the end of a column 2.
FIG. 11 is a lateral section of the commutator motor 1. This commutator 1 includes: a cylindrical yoke 4; 4-pole magnets 6 disposed opposite each other inside the yoke 4 and held by a magnet holder 5 shown in FIG. 12; a shaft 9 supported at one end by a first bearing 8 housed in a bearing housing portion 7 of the yoke 4 so as to rotate freely; an armature 10 secured to the shaft 9; a commutator 11 secured to one end of the shaft 9; brushes 13 placed in contact with the surface of the commutator 11 by the elastic force of springs 12; a brush holder 14 for holding the brushes 13; a housing 16 integrated with the yoke 4 by a bolt 15; a bolt 17 securing the brush holder 14 to the housing 16 by means of an elastic body 100 which is rubber; a second bearing 18 secured to a central portion of the housing 16 for supporting the other end of the shaft 9 so as to rotate freely; and a grommet 21 through which a lead wire 20 passes connected by welding to metal fittings (not shown) whose tips are insertion molded into the brush holder 14.
The armature 10 includes: a core 22 having a plurality of slots extending in the axial direction; and a winding 23 constructed by winding wire into the slots by a lap-winding method.
The magnet holder 5 includes: four stanchion portions having a plurality of protrusions 31 for holding the magnets 6; and an engaging portion 32 formed on a radially inner portion of the magnet holder for engaging a recess (not shown) in the brush holder 14.
In a commutator motor 1 of the above construction, the armature 10 is rotated together with the shaft 9 due to electromagnetic action by supplying electric current from the lead wire 20 to the winding 23 by means of the brushes 13 contacting the commutator segments 11. The torque of the shaft 9 is transmitted to a shaft 24 spline fitted to a boss 19 secured to the shaft 9, and serves to assist the steering force acting on the steering wheel 25.
The commutator motor 1 is mounted to a mounting seat 33 close to the vehicle cab, requiring strict noise prevention. Vibrations transmitted from the gear box 3 side are transmitted through the shaft 24 and the housing 16 to the brush holder 14, which causes the brushes 13 to vibrate, giving rise to brush noise, and the brush holder 14 is also supported on the housing 16 by means of the elastic body 100 in order to prevent the sliding noise of the brushes 13 from being transmitted from the brush holder 14 through the housing 16 to the gear box 3 side or to the yoke 4. That is to say, the brush holder 14 is rubber mounted.
However, in a commutator motor 1 of the above construction, one problem has been that since the lead wire 20 passes through the grommet 21 and extends outside as shown in FIG. 13, the brush holder 14 held by the elastic body 100 may be displaced from center or be tilted relative to the shaft 9 by movement of the lead wire 20 which is connected to the metal fittings inside the brush holder or during installation of the brush holder 14, making contact between the brushes 13 and the commutator 11 uneven, causing the performance of the commutator motor 1 to deteriorate (for example, rotational frequency and rotational torque may be different in the clockwise and counterclockwise directions), and shortening the working life of the brushes 13 and increasing brush noise. This problem is particularly serious in the case of a motor for an electric power steering assembly which rotates in both directions.
Another problem has been that although the brush holder 14 is supported on the housing 16 by means of the elastic body 100, because the brush holder 14 is positioned by the engaging portion 32 of the magnet holder 5, vibrations from outside, for example, may be transmitted to the brush holder 14 through the yoke 4, the magnets 6, and the magnet holder 5, which causes the brushes 13 to vibrate, giving rise to brush noise.
An additional problem has been that because the first bearing 8 is housed in the bearing housing portion 7 of the yoke 4, if there is any clearance between the bearing housing portion 7 and the first bearing 8, the first bearing 8 may knock against the inner wall of the bearing housing portion 7 when the armature 10 rotates, giving rise to impact noises, and brush noise may increase due to rattling of the armature 10.